This disclosure relates to a method for automatic detection of need for clutch calibration. The disclosure also relates to a clutch system that automatically detects need for clutch calibration. The disclosure is particularly advantageous in the field of friction clutch systems for vehicles.
In automated manual transmissions AMT having a single or double clutch the clutch is operated by an electromechanical clutch actuator and gear shift operations within the gearbox are performed by one or more electromechanical shift actuators. The electromechanical actuators are controlled by an electronic control unit, which controls the shift positions fully automatically. To ensure smooth and jerk-free gear changes it is important that the control unit has access to correct operating parameters of the clutch and transmission for controlling the electromechanical actuators using correct timing and power magnitude. Document US 2005/0192155 A1 for example concerns adaptation of engaging an automated clutch for the clutch of a new vehicle, or after clutch maintenance, service or repair. There is however always a risk that the adaptation of the clutch is not performed after clutch maintenance, service, or repair such that the driving comfort is reduced due to non-smooth gear changes. There is thus a need for an improved solution removing, the above mentioned disadvantages.
It is desirable to provide a method for detecting the need for clutch calibration, such that the previously mentioned problem can be avoided.
The disclosure concerns a method for automatic detection of need for clutch calibration. The method comprises the steps of:                registering and storing in a data memory, before or in connection with engine shut down, a value of a clutch operating parameter;        registering a corresponding value of the clutch operating parameter in connection with subsequent engine start up; and        determining need for clutch calibration if the difference between the stored value of a clutch operating parameter and the corresponding value of a clutch operating parameter exceeds a predetermined threshold value.        
The idea is to detect if the clutch characteristics have substantially changed between engine shut down and engine start up, by comparing a value of a clutch operating parameter stored before or in connection to shut down with a corresponding value of a clutch operating parameter registered at the subsequent engine start up. The reason for the clutch characteristics to substantially change between an engine shut down and an engine start up is usually that one or more plates of the clutch, such as one or more clutch plates, the pressing plate or the support plate, have been exchanged. Clutch plates wear during take-off and gear shifts and have to be regularly replaced. The replacement is done with the vehicle engine shut down. However, the replacement of a worn clutch plate with a new plate substantially changes the clutch characteristics. Primarily the clutch actuating positions are displaced, e.g. the clutch engagement or disengagement position at which the clutch starts and stops to transmit torque respectively, as well as the position where the clutch is fully engaged. Fully engaged refers to the maximum degree of torque transfer of a clutch in the specific clutch configuration of the drive train in question. The maximum degree of torque transfer depends for example on the magnitude of the clamping force that forces the clutch plates towards each other.
When the clutch characteristics have changed substantially, as is normally the case when a clutch plate has been replaced, clutch calibration is needed for adapting the control system to the increased clutch plate thickness, such that good driving comfort at gear shifts is obtained and such that the risk of the engine stalling or having high propulsion torque without accelerator actuation when a gear is engaged is avoided. Having high torque without accelerator actuation is highly dangerous since the vehicle unintentionally may move forwards or backwards, risking to hit someone or to run into something. If a clutch plate is replaced by an authorised technician, a subsequent clutch calibration process is generally commanded by the technician. However, there is always a risk that the authorised technician for some reason misses out the calibration, or that the replacement of the clutch plate is performed by a non-authorised person who does not have knowledge about the required calibration or for some other reason misses out the calibration. And, as explained above, missing out the clutch calibration after clutch plate replacement may have severe consequences. One advantage of the disclosed method is that it provides automatic detection of the need for clutch calibration at start up of the engine, thus reducing the risk of accidents and/or the engine stalling due to incorrect clutch calibration. Since a clutch plate exchange in theory could take place during any stand still, the method is preferably applied in connection to every engine shut down and start up.
The disclosure further concerns a corresponding computer program, a corresponding computer program product, and a corresponding computer system for implementing the method. The disclosure also concerns a corresponding clutch system comprising a friction clutch and an electronic control unit, wherein the control unit is programmed to automatically detect need for clutch calibration by performing the steps of:                registering and storing in a data memory, before or in connection with engine shut down, a value of a clutch operating parameter;        registering a corresponding value of a clutch operating parameter in connection with subsequent engine start up; and        determining need for clutch calibration if the difference between the stored value of a clutch operating parameter and the corresponding value of a clutch operating parameter exceeds a predetermined threshold value.        
Further advantages are achieved by implementing one or several features of other aspects of the invention.
The selected clutch operating parameter is preferably more or less sensitive to the wear condition of the clutch, in particular to a wear condition of at least one of a clutch plate, pressing plate or support plate. The more wear sensitive selected clutch operating parameter the more accurate and reliably can the need for clutch calibration be determined.
The value of the clutch operating parameter is preferably derived from a clutch actuating position along the clutch characteristic curve. The clutch actuating position is good and relatively easily registered indicator of the clutch wear condition. Preferably, the clutch operating parameter corresponds directly to the clutch actuating position along the clutch characteristic curve.
The stored value of the clutch operating parameter of the disclosed method may be derived from any of the clutch engaged position, the clutch engagement position, or the clutch disengagement position. In the clutch engaged position, the support and pressing plates of the clutch abut against each other, usually via an intermediate clutch plate, and the torque transmitting capability of the clutch is at its maximum. The clutch engagement position is the physical position at which an initially disengaged clutch being actuated starts to transmit torque. And vice verse, the clutch disengagement position is the physical position at which an initially engaged clutch being disengaged stops to transmit torque. The clutch engagement position and the clutch disengagement position may correspond to the same physical position of the pressing plate, but if hysteresis effects are present, they will correspond to separate positions. Preferably, the clutch engaged position is used in the disclosed method, since the clutch engaged position is quick and easy to find. Finding the clutch engagement or disengagement position is also possible, but requires for example a pressure sensor in connection to the clutch plates for detecting the clutch plate clamping pressure. In a clutch disengaged state, the clutch plate clamping pressure is zero. If starting in a disengaged state and moving the pressing plate towards the support plate, the clutch engagement position is reached at the moment when the pressure sensor detects a clutch plate clamping pressure larger than zero. In order to find the disengagement position, it is suitable to start in a clutch engaged state, where the clutch clamping pressure is large, and move the pressing plate way from the support plate. The clutch disengagement position is found as the pressure sensor detects that the clutch plate clamping pressure has reached zero.
Another possible way of finding the clutch engagement or disengagement positions is to use an angular sensor which is sensitive to the rotational speed of the input shaft of the transmission of the vehicle, i.e. the shaft downstream the master clutch of the drive train. The clutch engagement position can be found by the following procedure: Start with the clutch in its disengaged state and the input shaft stationary, i.e. non-rotating, and the crankshaft, i.e. the shaft upstream the master clutch, rotating. Move the pressing plate towards the support plate. As the clutch starts to transfer torque, torque will be transferred from the crankshaft to the input shaft which will start to rotate. Consequently, it can be concluded that the clutch has reached its engagement position at the moment the rotation sensor detects that the input shaft starts to rotate. For finding the clutch disengagement position, an additional rotation sensor sensitive to the rotational speed of the crankshaft is needed. Initially, the clutch should be in its engaged state and the crankshaft and the input shaft should rotate at a common speed separate from zero, i.e. the two rotation sensors should read the same value. The pressing plate is moved away from the support plate. As the clutch ceases to transfer torque, the input shaft will start to decelerate and its rotational speed will start to differ from the speed of the crankshaft. Consequently, it can be concluded that the clutch has reached its disengagement position at the moment the rotation sensors starts to detect different rotational speeds of the crankshaft and input shaft respectively. During the described procedures for finding the clutch engagement and disengagement positions by using rotation sensors, the input shaft has to be disconnected from the gear box output shaft in order for the vehicle to remain stationary. Alternatively, the stored clutch actuating position may correspond to a certain engagement pressure and/or a certain torque transfer value. The engagement pressure corresponds to the aforementioned clutch plate clamping pressure and can be measured with a pressure sensor in connection to the clutch plates. Said torque transfer value may be measured with a torque sensor, for example a strain gauge, mounted on the input shaft of the transmission. A strain gauge measures the deformation of the input shaft, from which deformation the torque is deduced.
The method may also comprise performing an automatic clutch calibration if a need for clutch calibration has been determined. The need for calibration is not just detected, but the required calibration is also completed. Hence, the aforementioned risks and inconveniences associated with an uncalibrated clutch are eliminated.
Furthermore, the method may comprise performing the automatic clutch calibration before enabling use of the clutch for transmitting driving torque. The advantage is automatic elimination of the aforementioned risks and inconveniences that comes from driving with an uncalibrated clutch.
The method may comprise performing the automatic clutch calibration by updating at least one stored clutch control parameter with a value corresponding to the determined difference between the stored value of the clutch operating parameter and the corresponding value of the clutch operating parameter. This is equivalent to adapting the clutch control parameter to the new clutch characteristics. For example, the stored clutch control parameter may be any of the stored clutch engagement position, the stored clutch disengagement position or the stored clutch engaged position. Preferably, all these clutch control parameters are updated during the clutch calibration.
Alternatively, the method may comprise performing the automatic clutch calibration by updating, and displacing a stored complete clutch characteristic curve. A clutch characteristic curve comprises all possible clutch actuating positions of the clutch. Consequently, this provides a more comprehensive calibration of the clutch than updating separate clutch control parameters.
Instead of automatic clutch calibration, the method may comprise signalling clutch calibration need to a driver of the vehicle and/or sending information concerning clutch calibration need to a remote party if a need for clutch calibration has been determined. This might be a good alternative if for example automatic clutch calibration is not available for the vehicle. The driver or a remote party such as vehicle fleet management or vehicle producer becomes aware of the fact that the clutch needs calibration, and may take necessary action to ensure that clutch calibration is indeed carried out.
If a need for clutch calibration has been determined the clutch control may alternatively, or preferably in combination with said signalling and/or sending information concerning clutch calibration need, be set in a safety operation mode where the output signal from the position sensor is disregarded. Thereby the vehicle can be used with a degraded performance despite the need for a clutch calibration but without the imminent risk for accidents and discomfort.
The clutch actuating position may be registered by a position sensor that is arranged to register the position of an axially displaceable pressure plate or a member of a clutch actuating mechanism that is arranged to generate the axial displacement of the pressure plate. This can be accomplished in several ways. The position sensor may either be integrated in the clutch actuating mechanism, or operated as a standalone element that senses either the position of the pressure plate directly or indirectly by sensing the position of the clutch actuating mechanism member, e.g. a piston rod that acts upon the pressure plate.
The method may comprise performing at least one additional disengagement-engagement sequence if the difference between the stored value of the clutch operating parameter and the corresponding value of the clutch operating parameter is within a predetermined range, and subsequently registering a new corresponding value of the clutch operating parameter. If the clutch has been removed and put back again, without the exchange of any plates, the position sensor of the clutch may read a more engaged position than it should. This may result in a difference between the stored clutch actuating position and the corresponding clutch actuating position within a predetermined range. The predetermined range may for example be 0.3-2 millimeters, preferably 0.3-1.5 millimeters, and more preferably 0.3-1 millimeters. If the detected difference in clutch actuating position is within the predetermined range, this is an indication of that no plate has been exchanged, but merely that the existing plates and/or the position sensor have been brought slightly out of position. To correct the reading of the position sensor, the clutch is disengaged/engaged a number of times in order to bring the clutch components and the sensor to their correct positions. After that, a corresponding clutch actuating position is registered again. The advantage of additional disengagement-engagement sequences is thus to correct deviations in the clutch position reading emanating from the act of removing and reinstalling the clutch. All absolute values relate to the position of the pressing plate.
The method may further comprise registering the value of the clutch operating parameter at each clutch engagement and/or disengagement occurrence. In such way, a fairly recent value of the clutch operating parameter is always stored at engine shut down, no matter when and how the engine shut down takes place and no additional measurement of the value of the clutch operating parameter must be performed in connection to engine shut down. The alternative is to have a specific procedure for registering and storing the value of the clutch operating parameter just before or concurrent with engine shut down.
The method may also comprise regularly updating the stored value of the clutch operating parameter based on an average value calculated using a set of recently registered values of the clutch operating parameter. Using an average value reduces the effects of measurement errors and white noise in the stored value of the clutch operating parameter.
The predetermined threshold value used for determining the need for clutch calibration may be larger than 1 millimeter, preferably larger than 2 millimeters, more preferably larger than 3 millimeters, and even more preferably larger than 4 millimeters. A higher threshold increases the risk of failing to discover that a clutch plate replacement has been performed and that a clutch calibration is required. A lower threshold increases the frequency of calibrations although no clutch replacement was performed. All absolute values relate to the position of the clutch actuator.